1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrostatic process or an electrophotographic recording process, etc., and particularly relates to an image forming apparatus that uses an intermediate transfer member onto which a developed image is primarily transferred and from which the developed image is secondarily transferred onto a transferring material.
2. Related Background Art
Conventionally, there have been used image forming apparatus provided with a plurality of image forming portions, each of which irradiates a laser beam or a light beam from a light emitting element such as an LED that is light-modulated based on image information onto an image bearing member such as a photosensitive drum to form an electrostatic latent image in accordance with an electrophotography process, then develops the electrostatic latent image by developing means accommodating developer to form a developed image (or a toner image), and transfers it onto a transferring material conveyed by a transferring material conveying member or an intermediate transfer member.
In addition, there have been proposed image forming apparatus for forming color images by forming toner images of different colors by means of the aforementioned plurality of image forming portions respectively and transferring the toner images of the respective colors onto a transferring material in a multi-layer manner while conveying the transferring material to the positions opposed to the respective image forming portions by means of a transferring material conveying member, or by transferring the toner images of the respective colors onto an intermediate transfer member in a multi-layer manner and then transferring them onto a transferring material at one time (intermediate transfer method).
In many cases, an endless belt that is looped around a driving roller for transmitting drive and at least one driven roller so that its surface will be moved is used as the intermediate transfer member. In this specification, endless belts serving as intermediate transfer belts will be collectively referred to as “transferring belts.” In addition, since a photosensitive drum is often used as the image bearing member, the image bearing member will be referred to as “a photosensitive drum.”
In the above-mentioned type of image forming apparatus, in order to improve transfer latitude (or transfer efficiency) upon transferring from the photosensitive drum to the transferring belt, it is considered to be effective to set a primary transferring current optimally. However, this involves difficulties since a transfer error tends to occur when the primary transferring current is low and re-transfer tends to occur when the primary transferring current is high.
In view of the above, it is a common practice to create a difference in peripheral speed between the photosensitive drum and the transferring belt in order to improve the primary transfer latitude. According to a presently proposed technology, by virtue of the peripheral speed difference, transferring is performed taking advantage of a shear force functioning to scoop the toner image on the photosensitive drum, so that an improvement and a stabilization of the primary transfer latitude upon primary transfer of the toner image on the photosensitive drum are attained and “uneven density” in images and “voids” in lines or character images are prevented from occurring. With this technology, voids can be prevented especially in the central portion of thin lines of a secondary color and an improvement in transfer latitude can be expected.
However, in the case that a peripheral speed difference always exists between the photosensitive drum and the transferring belt, a frictional force is present between them.
Consequently, the coefficient of friction varies depending on presence/absence of toner between the photosensitive drum and the transferring belt, and therefore the rotation speed of the photosensitive drum varies. As a result, image exposure on the photosensitive drum is blurred, and streaked images are sometimes generated at the leading edge portion of an image.
In connection with this, Japanese Patent Application Laid-Open No. H11-52758 discloses a structure for an apparatus that forms images while performing a control to make the surface speed of a photosensitive member and the surface speed of an intermediate transfer belt equal to each other. In the structure disclosed in this document, a dot toner image is formed on a drum before the first toner image is primarily transferred, in order to eliminate the problem that a misregistered image can be generated due to a partial speed difference that is abruptly generated between the photosensitive member and the intermediate transfer belt by an effect of the stress between those members that is created due to eccentricity of a roller on which the belt is looped or by a mounting error of those members.